Up to now, the powder core using soft magnetic materials is being developed as the magnetic core for the motor, actuator, generator, reactor or the like. Generally speaking, the magnetic core prepared by compressing the powder has a relatively low mechanical strength or a relatively low magnetic flux density compared to the silicon steel plate. As a producing method for overcoming these problems, it has been proposed a scheme in which the pressure for the molding process or the temperature in the heat treatment is elevated.
However, the powder core prepared by such treatments has a low electrical resistivity as the insulating film formed on the surface of the particle is likely to peel off or break up. The eddy current in the magnetic core will increase if the electrical resistivity is lowered. In this way, the output or the efficiency of the product will also decrease. Therefore, there is no soft magnetic powder core that can possess a high electrical resistivity, a high magnetic flux density and a high strength at the same time.
In order to solve these technical problems mentioned above, Patent Document 1 has disclosed a technology that the iron powder on which a coating film containing MgO is formed and the silicon resin are mixed and molded to give a powder core, and the molded powder core is subjected to a firing process at a temperature of 550° C. to 750° C. under a non-oxidative atmosphere followed by a heat treatment at a temperature of 400° C. to 560° C. under an oxidative atmosphere. In addition, Patent Document 2 has disclosed a technology that a compound or salt which generates a cation having a valence of 2 or more with boric acid or phosphoric acid is used to form an insulating coating film on the iron powder. Further, a technology has been disclosed in Patent Document 3 that soft magnetic particles with an insulating coating film, glass with a low melting point and an average particle size of 2 nm to 200 nm, and a lubricant are mixed and compressed and then a firing treatment is provided to the mixture at a temperature of 650° C.